This invention relates to a modem device for use in a multivalued quadrature amplitude modulated signal communication network in carrying out modulation and demodulation between a multivalued quadrature amplitude modulated signal and input and output digital signal sequences. This invention relates furthermore to a pair of surface acoustic wave (SAW) filter circuits for use in the multivalued quadrature amplitude modulated signal communication network.
In the manner which will later be described in detail, a modem device comprises a modulator device and a demodulator device. A multivalued quadrature amplitude modulated signal communication network comprises such a modem device as a first modem device and a counterpart modem device which may be called a second modem device.
According to prior art, the modulator device of each modem device comprises a digital-to-analog converter circuit for converting a plurality of input digital signal sequences to a plurality of multivalued baseband signal sequences, respectively. After band-limited, the baseband signal sequences are used in quadrature amplitude modulating a modulator carrier signal into a plurality of amplitude modulated signal sequences, respectively. A combining circuit is used in combining the amplitude modulated signal sequences into a multivalued quadrature amplitude modulated signal.
Such multivalued quadrature amplitude modulated signals are produced by the first and the second modem devices as first and second modulator output signals. It is unnecessary that the modulator device of the first modem device should deal with the input digital signal sequences of a number which is equal to the number of input digital signal sequences used in the modulator device of the second modem device. In any event, the first modem device receives the second modulator output signal from the second modem device as a first demodulator input signal. The second modem device receives the first modulator output signal as a second demodulator input signal.
In each demodulator device, such a demodulator input signal is branched by a branching circuit into a plurality of branched signal sequences. In a conventional demodulator device, the branched signal sequences are rolloff waveform shaped into a plurality of rolloff waveform shaped signal sequences, respectively. A quadrature amplitude demodulator circuit is used in quadrature amplitude demodulating the rolloff waveform shaped signal sequences into a plurality of demodulated signal sequences, respectively. The demodulated signal sequences are converted by an analog-to-digital converter circuit to a plurality of output digital signal sequences, respectively. The demodulated signal sequences are reproductions of the respective multivalued baseband signal sequences used in the counterpart modem device which produces the demodulator input signal under consideration. The output digital signal sequences are reproductions of the respective input signal sequences used in the counterpart modem device in question.
For band limitation, a first low-pass rolloff filter has been used in the modulator device of each modem device. In the demodulator device of each modem device, rolloff waveform shaping has been carried out by a second low-pass rolloff filter in cooperation with the first low-pass rolloff filter used in the modulator device of the counterpart modem device.
The multivalued quadrature amplitude modulated signal communication network must have an excellent transmission characteristic. Each demodulator device must produce the output digital signal sequences with a low bit error rate. In order to achieve the excellent transmission characteristic and, in particular, to reduce intersymbol interference and thereby to attain a least possible bit error rate, the rolloff filters must have strictly limited passband distortion and delay distortion. More specifically, the rolloff filters must have passband amplitude and delay characteristics of least possible deviations and have a steep out-of-band attenuation characteristic.
Such stringent filter characteristics have recently become more stringent in order to deal with a highly multivalued quadrature amplitude modulated signal and to cope with a lower rolloff rate. The low-pass rolloff filters must therefore be higher-order filters, each of which comprises many capacitors and coils and is bulky and expensive.
In view of the above, a surface acoustic wave filter has recently been used as a rolloff filter. Inasmuch as the surface acoustic wave filter is used in general as a band-pass filter, it is possible to rolloff waveform shape a filter input signal in a carrier frequency band. The surface acoustic wave rolloff filters of a half in number are therefore sufficient in each modem device as compared with the low-pass rolloff filters. Moreover, each surface acoustic wave rolloff filter is compact when compared with each low-pass rolloff filter. This makes it possible to provide a compact modem device.
A conventional surface acoustic wave rolloff filter has, however, a passband delay characteristic which has ripple components. The bit error rate is deteriorated by the ripple components. Low-pass rolloff filters are therefore used even in a recent modem device, particularly when the modem device is used in dealing with a highly multivalued signal of, for example, two hundred and fifty-six values.